GENERATION AND STABILITY OF BENTONITE COLLOIDS

INTRODUCTIONIn Finland, the repository for spent nuclear fuel (SNF) will be excavated at a depth of about 500 meters in the fractured crystalline bedrock. The repository holes will be compacted with bentonite clay. The bentonite is assumed to be a potential source of colloids which could enhance the migration of radionuclides in the case of canister failure. The potential relevance of the colloids for radionuclide transport is highly dependent on their release and stability in different chemical environments and their interaction with radionuclides. [1] As the bentonite barriers are a critical part of SNF disposal concept, investigation of the processes that control bentonite erosion, clay colloid generation and stability under different chemical conditions is essential to ensure safety.

EXPERIMENTAL• MX-80 Volclay bentonite

(76 % montmorillonite) and Nanocor PGN Montmorillonite (98 %)

• OLSO (I = 0.517 M) and Allard (4.2 mM) reference groundwater

• NaCl, CaCl2 and diluted OLSO (1 mM – 0,1 M)

• 2 g bentonite powder or 2 pellets and 45 mL solution → Samples stored with gentle agitation or without agitation → Centrifugation

• Particle size, concentration and zeta potential

• The solutions were changed and the samples were put back to the agitation

• The measurements were repeated at an interval of approximately 50 days

• Photon correlation spectroscopy (Malvern Zetasizer Nano ZS)

• The stability of bentonite colloids was also investigated in the long term experiment with a sampling interval of a year

RESULTS The size of bentonite colloids increased as the ionic strength of the solution increased. Similarly, the zeta potential approached zero as the ionic strength increased.

The ZP’s were under -20 mV in dilute solutions indicating the stable colloids.In more saline solutions, the particle size distribution was wide and varied from a few nanometers to thousand of nanometers. The ZP’s were close to 0 mV.

The colloid formation was dramatically increased with the agitation. Although, the colloid formation was significantly decreased after the first sample measurement and changing of the solutions.

CONCLUSIONSThere can be seen a clear difference between saline Olso and more diluted samples.

The colloid release was notable in the beginning of the contact in dilute solutions but decreased evidently.

The stability of bentonite colloids depends strongly on the ionic strength and the valence of the cations in the solution.The colloid dispersion has remained stable in low salinity solutions so far over four years.

At saline conditions in Olkiluoto, colloids are unstable → No effect on the radionuclide transport. The possible post-glacial dilute groundwater implies that the colloids may have to be taken into account.

REFERENCE[1] Schäfer T. Huber F, Seher H, Missana T, Alonso U,

Kumke M, Eidner S, Claret F and Enzmann F (2012). Nanoparticles and their influence on radionuclide mobility in deep geological formations. Appl. Geochem., 27(2), 390-403.

Fig. 3. Mean cumulative particle concentration of colloids formed from MX-80 bentonite pellets and Nanocor PGN powder in different water solutions.

Fig. 4. Mean zeta potential of colloids formed from MX-80 bentonite pellets and Nanocor PGN powder.

Fig. 5. Mean particle size of colloids formed from MX-80 bentonite pellets and Nanocor PGN powder.

Valtteri Suorsa, Pirkko HölttäLaboratory of Radiochemistry, Department of Chemistry

University of Helsinki, Finland

These results have received funding from the European Atomic Energy Community’s (EURATOM) 7th Framework Programme (FP7/2007-2011) under the grant agreement No. 295487, the BELBaR project.

Fig. 1. Colloid/radionuclide and host rock interaction. (EU/BELBaR/T. Schäfer)

Fig. 2. Formation of bentonite colloids in different water solutions. Allard, OLSO, OLSO 5 mM, OLSO 1 mM, NaCl & CaCl2 1 mM, respectively.

Fig. 6. Mean zeta potential of colloids formed from MX-80 bentonite powder in diluted Olso reference groundwater.

Fig. 7. Mean Zeta potentials of colloids formed from Nanocor powder in the different dilutions of OLSO.